support/testing: test_python_django: use the timeout_multiplier value

support/testing: test_python_django: use the timeout_multiplier value

As reported by our gitlab runtime test [1] and on the mailing list
[2], the test_python_django is failing due to django server taking a
lot of time to start. Since the django server is started in background
through pexpect, we can't easily wait for the last startup line:

"January 01, 1970 - 00:00:41

Django version 3.0.4, using settings 'testsite.settings'

Starting development server at http://0.0.0.0:1234/

Quit the server with CONTROL-C."

In the failing gitlab job, we don't see such lines.

If we increase a lot the timout, the test passes.

Use timeout_multiplier introduced by [3] in order to adjust the
timeout.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/488816222

Tested:
https://gitlab.com/kubu93/buildroot/-/jobs/507458355

[1] https://gitlab.com/buildroot.org/buildroot/-/jobs/488816222
[2] http://lists.busybox.net/pipermail/buildroot/2020-April/279598.html
[3] 6e45e33f27d5ae6fa0ab5aad3f032d886a886037

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>